A new study found that police in British Columbia were far more likely to document alcohol involvement in a crash than the involvement of cannabis, other recreational drugs, or potentially impairing medications. The study’s authors suggest this raises doubts both about the police’s ability to enforce drug-impaired driving laws as well as public health officials’ documentation of drug-impaired driving. As jurisdictions legalize marijuana usage for recreational purposes, they will need to ensure that existing laws surrounding driving under the influence fully apply to cannabis.

Canadian researchers obtained blood samples and police reports for 1,816 injured drivers treated in British Columbia trauma centers following a crash. They analyzed the blood samples for alcohol, cannabis, other recreational drugs, as well as other potentially impairing medications. The researchers then examined the corresponding police report to see if any substances found in the blood sample were documented in the report as impairing the driver’s abilities and/or were a contributing to the accident.

Fifteen percent of the blood samples (272 drivers) tested positive for alcohol but only 64 percent of the corresponding police reports documented this. Alarmingly, 20 percent of the samples tested positive for impairing medications but this was only documented in 2.2 percent of the corresponding police reports. For cannabis, 7.5 percent of the blood samples tested positive but only 5.9 percent of the corresponding police reports indicated this. Finally, for other recreational drugs, 9.1 percent of the blood samples tested positive, but this was only indicated in 16.9 percent of the corresponding police reports.

The report’s authors express concern about the ability of police, at least in British Columbia, to enforce existing impaired driving laws. They also note that public health officials’ use of police crash reports to understand drug-impaired driving may lead to incomplete conclusions. While Canada is in the process of legalizing marijuana for recreational usage, impaired driving will remain illegal.

In the United States, it is illegal in every state to drive while impaired by any drug, in the same way that driving while impaired by alcohol is illegal. The driver must both demonstrate their impairment as observed by a law enforcement officer and that impairment must be linked to a drug. Additionally, at least sixteen states have zero tolerance laws for specified drugs—generally excepting prescriptions. Finally, at least seven states have per se laws, making it illegal to drive with amounts of cannabis and/or other drugs exceeding a specified limit.

As the recreational use of marijuana becomes more widespread across the United States and Canada, enforcement of laws governing driving under the influence will need to keep up with this trend.

For example, Colorado in 2012 became the second state in the U.S. to legalize the sale and possession of marijuana for non-medical use. Colorado’s per se law states that drivers with five nanograms of active tetrahydrocannabinol (TNC) can be prosecuted for driving under the influence. This is true whether the cannabis was consumed recreationally or for medicinal purposes. The state’s open container law also applies to marijuana in the same way as it applies to alcohol.

Laws governing driving while under the influence of alcohol have been around for decades. As legal marijuana usage becomes more prevalent, the British Columbia study shows that law enforcement officers will need to become more adept at recognizing and testing for the involvement of cannabis in both accidents and driving under the influence cases. The same is true of other drugs, both legal and illegal.